Prostaglandin (PG) signaling pathway has long been implicated in various female reproductive functions including implantation, because of vasoactive, mitogenic and differentiating properties of PGs. Indeed, expression and gene targeting experiments have established a fundamental role for cyclooxygenase-2 (COX-2), the rate-limiting enzyme in PG biosynthesis, in ovulation, fertilization, implantation and decidualization. Further experiments revealed that prostacyclin (PGI2) is the major product of uterine COX-2, and among several receptor candidates, this PG utilizes PPARdelta in mediating implantation. This work demonstrated the first biologic function of PPARdelta and showed that the mouse is a powerful model to scrutinize the molecular and physiological mechanism of PPARdelta-driven implantation process. The PPAR family of transcription factors belongs to the nuclear hormone receptor superfamily. Transcriptional activation of target genes by PPAR depends on ligand-induced heterodimerization with a retinoid X receptor (RXR) and recruitment of several cofactors. Thus, uterine PPARdelta, when bound to COX-2-derived PGI2, is likely to regulate the expression of a bank of genes involved in implantation. Among the three isoforms of PPAR, PPARalpha and PPARgamma are implicated in adipocyte differentiation, fatty acid metabolism, or terminal differentiation of cancer, while information on PPARdelta is limited. Therefore, an in-depth investigation on the molecular pathway of PPARdelta is proposed in a physiologically relevant system, i.e., embryo implantation. Our specific aims using the mouse are: 1) to examine whether COX- 2 is regulated by PPARdelta during implantation; 2) to identify cofactor usage by uterine PPARdelta/RXR; 3) to identify downstream target genes of PPARdelta in the uterus by chromatin precipitation; and 4) to test the involvement of PPARdelta in implantation using PPARdelta deficient mice and with mouse models with directed expression of dominant-negative PPARdelta in the uterus. Experimental approaches includes the use of mutant mice, Northern and Western blotting, in situ hybridization, co- immunoprecipitation, cell culture and transfection, chromatin precipitation, protein delivery system and generation of transgenic mice. The results obtained from these studies will provide novel information regarding the unique role of PPARdelta and will advance our understanding in human reproduction with potentials in alleviating female infertility and development of contraceptives.